Defect of the Month – Low Temperature Soldering

Hello, my name’s Bob Willis and welcome to Defect of the Month. This month I wanted to talk about low temperature solder alloys and low temperature soldering. These solders have become quite popular to decrease energy consumption, prevent warpage and distortion of boards/components. They allow us to reduce the cost of manufacture based on materials and obviously energy consumption. So there’s are a number of advantages to consider. The low temperature materials tend to be tin/bismuth or tin/bismuth/silver alloys. Some standard alloys have been modified to improve the long-term reliability of solder joints. The standard options reflow at around 140degC. The materials which are slightly higher reflow temperatures have demonstrated higher reliability. Tin/bismuth silver materials are readily available in bulk alloy for selective and wave soldering. 

In addition, the low temperature materials are available in solder paste for reflow soldering but are limited in cord wire for hand soldering and rework and repair. But they are available if you search them out. The main reason there is limited availability is the brittle nature of tin/bismuth solder alloys. They are difficult to draw to the wire gauges ideally required in electronics manufacturing, ideally 0.6 – 1mm

Now what I wanted to talk about on this particular video are solder balls. I haven’t really had any problems with reflow of paste. I also haven’t had too many problems with selective and wave soldering. But where I’ve been using cord wire with low temperature materials, particularly on robotic soldering, I’ve noticed quite a high level of soler balling. 

Examples of solder balling due to spitting during the soldering operation

Now this is a process issue, something you can look at, modify and overcome. It’s not a robotic soldering problem, but it depends on how you implement the technology with any contact soldering operation. The key thing is making sure operators are trained to use solder wire with the correct feed rate and the temperature of the soldering iron must be set correctly. Trying to go fast, faster and faster because with some cored alloy, you might find that the solder spits resulting in solder balls. We have seen this in the past during the implementation of lead-free technology and other technologies you may well find more evidence of spitting with certain wire brands but it’s how you use them. 

If you add cored solder wire directly to the tip in a robotic soldering situation, you may see flux and solder spitting leaving solder balls on the product, you don’t want to have random balls that are not held in place. Random solder balls can be anywhere on the assembly which would not be acceptable in the automotive industry or to IPC. There are a couple of things to consider first look at the process, look at preheating that you could introduce, which a lot of robotic soldering systems don’t have. You can use preheating from the blanket of nitrogen that you introduced around the soldering iron tip. This can actually improve temperature balance and the degree of spitting you might see and may make the process slightly more efficient as well. But one of the things that I’ve found that is the most beneficial is scoring the wire into the flux core 

Example of cored wire after indenting with the feed wheel. The x-ray image shows the wire

Some robotic soldering systems allow you to score or indent the surface of the wire. This prevents explosions of solder when you are going through the robotic soldering operation, and I’ve illustrated this on video clips during this online video presentation. Now what we’re talking about is just indenting into the wire as part of the process. This is part of the process of feeding/indexing the wire in the first place in an automated system. If you indent into the wire and the indent goes into the core of the flux within the wire itself. When the wire goes from a solid state into a semi-liquid, there’s not that explosion of energy. The flux can escape from the surface before it actually becomes a liquid. Consequently, you are much less likely to get that explosive reaction. So as part of your trials speak to your supplier, evaluate the materials to start off with and look to that function of indenting wire. It certainly does improve performance and the incidence of solder balling quite significantly. My name’s Bob Willis with another Defect of the Month, and I’ll see you next time